Studies initially pioneered in my lab and subsequent work by us and others established that IL-2 is an essential cytokine for the development, homeostasis and function of regulatory T cells (Tregs). We have further established in mouse models that low levels of IL-2R signaling effectively promote Treg but not T effector (Teff) cells. These findings have helped in the clinical translation of low-dose IL-2 as a Treg-targeted therapy for autoimmunity. Moreover, serious autoimmune-related side effects frequently occur as a consequence of checkpoint blockade during cancer immunotherapy, raising the possibility that low-dose IL-2 might alleviate these unwanted responses. In spite of this progress, we still have a poor understanding of the mechanisms by which IL-2 controls peripheral Tregs. Recently, we have developed several novel mouse models that vary IL- 2R signaling in Tregs. In one model (NOD-Y3), IL-2R signaling strength was lowered approximately 3-fold in Tregs in NOD mice, which accelerated diabetes. In another model, CD25 (IL-2R?) was selectively abrogated in peripheral Tregs (CD25iKO mice), where all peripheral Tregs require IL-2R signaling for their survival. Our genome-wide expression studies suggests homeostasis of Tregs is impaired in the absence of IL-2R signaling due to decreased cholesterol biosynthesis that alters Treg metabolism and to lower expression of anti- apoptotic Bcl-2 that favors apoptosis. Although Treg suppressive function depends on IL-2R signaling, definitive data for this notion comes from studies where IL-2R signaling is impaired during thymic development. The suppressive activity of peripheral Tregs that have not responded to IL-2 remains unknown. Moreover, current models suggest that IL-2R signaling is primarily required for the homeostasis of resting or central Tregs (cTregs) while TCR signaling controls activated or effector Treg (eTregs). However, we found that the eTreg subset cannot survive without IL-2. Thus, the role of IL-2 for eTregs remains unclear. Our studies also indicate that reduced, but not absent, IL-2R signaling at the level of Tregs is sufficient to promote autoimmune- mediated diabetes in NOD mice. However, IL-2-dependent processes that are impaired within the islets of the pancreas to accelerate diabetes are poorly understood. We plan to build on these results and capitalize on our expertise on the IL-2R to address these issues and better understand mechanistically the basis by which IL-2R signaling regulates Treg homeostasis and promotes susceptibility to autoimmunity. The following aims are proposed: 1) To establish the gene signatures for peripheral Treg subsets that depend on IL-2R, STAT5 and mTORC1 signaling; 2) To determine the relative roles of IL-2R, STAT5 and mTORC1 signaling for survival and metabolic function, including the contribution of lipid biosynthesis and Bcl-2, for the homeostasis of peripheral Treg subsets; 3) To define the extent absent IL-2R, STAT5, and mTORC1 signaling alters Treg function and to determine the cellular and molecular basis by which reduced, not absent, IL-2R signaling in Tregs promotes autoimmunity in NOD mice.